The field of this invention is seismic telemetry systems.
In seismic survey operations, a seismic signal is imparted to the land surface within the survey area at a shot point. The seismic signal travels from the shot point downwardly from the land surface until it is at least partially reflected by geological strata beneath the land surface. A plurality of seismic signal detectors or detector groups, each of which is positioned at a distinct detector station, senses the reflected seismic signal at that station and produces an electrical output signal corresponding to the detected seismic signal. The outputs of the detectors or detector groups are sampled for a period of time subsequent to sending the seismic signal to the land surface, and these outputs are recorded by a seismic recorder for subsequent processing and interpretation. The shot point and detector stations are then relocated and the process is repeated until a sufficient amount of data has been collected within the survey area.
In many survey systems, each of the detectors or detector groups was electrically connected to the seismic recorder by a cable which included a conductor running from each detector or detector group to the recorder. When the number of detectors to be used with the system was increased, the number of conductors which had to be provided in the cable also had to be increased. As a consequence, the cables were often quite complex in structure and difficult to maintain and repair. Further, when seismic survey operations had to be conducted in areas having a rugged terrain, the cables were quite difficult to handle and were often damaged when they were handled. As a result, considerable expenditures of time, money, and manpower were required to maintain and repair these cables.
Known seismic telemetry systems eliminated the necessity of employing a cable by providing devices which could transmit seismic data directly from the detectors to the seismic recorder. However, the known seismic telemetry systems were not widely used in some areas of the world. With these systems, an extremely large amount of seismic data was transmitted to the recorder during the relatively short sampling period. A large number of channels were used to transmit the voluminous data within this period of time, necessitating the use of transmission equipment with extremely broad bandwidths. This, of course increased the cost of the equipment employed with such seismic telemetry systems. Further, in many areas of the world, licenses to broadcast data over such broad bandwidths could not be obtained from the local authorities. As a result, these known telemetry systems could not be used at many survey locations.